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CHAPTER-1
1
INTRODUCTION
1.1 General Introduction
Dielectric study of liquid mixtures has gained importance because it provides
one way of bringing together the molecules of different compounds and allows them to
interact with one another. The nature of complex formation in binary mixtures is still far
from clear. Dielectric investigations of solutions containing varying amounts of
interacting molecules help to detect the formation and composition of complexes in
them. Solids and liquids have been subjected to dielectric measurements at different
frequencies and temperatures since the properties shown are of interest and lead to a
knowledge of the composition and molecular structure of the material.
The interaction of electrical energy with matter is determined by the
electromagnetic properties of the material. On a macroscopic scale, under steady state
conditions, these properties are conventionally described by the permittivity and
permeability of the material. The increasing use of microwaves in fields like
communication, radar, medicine, biology, agriculture and industry demands accurate
data on dielectric properties of materials. The characterization of dielectric materials
includes the measurement of complex permittivity as a function of frequency at a given
temperature or as a function of temperature at a given frequency. The measurement of
dielectric properties over a wide frequency range gives the information regarding the
conduction mechanism, interfacial polarization, molecular dynamics and relaxation
behavior phenomena [1].
Hydrogen bonding is complex in the liquid state due to the uncertainty in
identifying the particular bonds and the number of molecules involved. Study of
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hydrogen bonding between –OH and –NH groups has been selected as it has extensive
applications in the biological research. The chosen alcohols and amines are propan-1-ol,
propan-2-ol and aniline and N-methyl aniline. In the present study, three hydrogen
bonded systems are investigated. To understand the molecular behaviour of associating
molecules, it is necessary to determine the various dielectric parameters which are
related with inter and intramolecular association and internal rotations with temperature
variation.
Determination of Kirkwood correlation factor g [2, 3] provides the information
about intermolecular association, while the anomalous behaviour of molar polarization
[4] shows the intramolecular association in the associating molecules. Dielectric
relaxation times [5, 6] give confirmation about intramolecular rotations and steric
hindrance offered by the intramolecular association and the environment to group
rotations. One of the physical parameters used for conformational analysis of any
resultant structure is the net dipole moment. Dielectric studies are carried out on the
binary mixtures to determine the dipole moment and relaxation time.
The aim of the present work is to understand the behavior of hydrogen bond
between the binary mixtures of alcohol and amine affecting the physical properties of
the systems such as dipole moment and relaxation time compared to the individual
systems since hydrogen bond involves –OH and –NH as functional groups. The
interaction between the molecules can be studied by using the quantum mechanical
calculations such as ab-initio and semiempirical methods. These quantum mechanical
calculations provide the information regarding the energy, nature of chemical bond,
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dipole moment, vibrational frequencies, optimized geometric structure of the molecule
and thermodynamical parameters.
Dielectric studies are of great help in the assessment of the molecular structures
or configurations, particularly those of organic compounds. Although IR and NMR
studies are powerful tools for studying intramolecular H-bonds, dielectric studies
provide useful information about molecular association and intramolecular rotations [5].
1.2 The Hydrogen Bond
Hydrogen bonding is a donor-acceptor interaction specifically involving
hydrogen atoms. A hydrogen bond is a link of the form A-H…B as described in Figure
1.1, where A and B are electronegative atoms and B has a lone pair of electrons. The
most significant hydrogen bonding occurs where A and B are atoms of Nitrogen (N),
Oxygen (O) and Fluorine (F). Weak hydrogen bonding can also be formed between less
electronegative atoms.
The simplest picture of a hydrogen bond is an electrostatic interaction between
the proton of A-H and the lone pair of B.
Figure 1.1: Hydrogen bond
The atom A needs to be electronegative in order to polarize the A-H bond and
hence leaves the charge of the proton partially unshielded. The atom B needs to be
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electronegative such that it has restrained a high electron density in the molecule of
which it may be a part and hence can interact reasonably strongly with the proton’s
exposed charge. In molecular orbital terms, a hydrogen bond consists of four electrons
in the three molecular orbitals that can be built from the appropriately directed atomic
orbitals of A, H and B.
Hydrogen bond energies extend from about 15-40 kcal/mol for strong hydrogen
bonds, 4-15 kcal/mol for moderate bonds and 1-4 kcal/mol for weak hydrogen bonds.
Strong hydrogen bonds are formed by groups in which there is a deficiency of electron
density in the donor group like O+, N+ or an excess of electron density in the acceptor
group like F-, O- and N-. This is to be expected since a deficiency of electrons of the
donor group further deshields the proton thereby increasing the positive charge. On the
other hand, while an excess of electrons on the acceptor group increases its negative
charge and interact with the deshielded proton. These bonds are also referred as ionic
hydrogen bonds.
Moderate hydrogen bonds are formed generally by neutral donor and acceptor
groups like –OH, –NH, O=C, in which the donor atoms (A) are electronegative relative
to hydrogen and the acceptor atoms (B) have lone pair unshared electrons. These are the
most common hydrogen bonds both in chemistry and nature. These are regarded as
normal hydrogen bonds and are essential components of the structure and function of
biological molecules.
Weak hydrogen bonds are formed when the hydrogen atom is covalently bonded
to a slightly more electro neutral atom relative to hydrogen, as in C–H, or when the
acceptor group has no lone pairs but has π electrons such as an aromatic ring. These
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interactions have similar energies and geometries to those of van der Waals complexes,
and are distinguished from them by the evidence of a directional involvement of A-H
bond.
The concept of hydrogen bond, naturally, evolves from consideration of
Pauling’s atomic electronegativities. As a consequence of the greater electronegativity
of A relative to H in an A-H bond, the hydrogen proton is stripped of some of its
electron density and is descreened. This results in a dipole at the terminus of the A-H
bond, which interacts with the monopole or dipole of the lone pairs on the acceptor
atom and to a lesser degree with more distant bond dipoles. Consequently hydrogen
bond donor strengths are qualitatively proportional to these differences in
electronegativities. F-H > O-H > N-H > C-H, and the hydrogen bond has a directional
property, being the strongest when A-H…B = 180°. The angles are easily bent from
linearity for moderate and weak bonds. Bent bonds are entropy favored. The peak in the
hydrogen bond angle distribution curve is ~155°. This is known as the conic factor or
correction. The other components of the hydrogen bond energy are identified as
delocalization, repulsion and dispersion. Coulson [7] partitioned the hydrogen bond
system into contributions from the five valence bond structures shown below.
ψ HB = a ψ a+ b ψ b + c ψ c + d ψ d+ e ψ e
ψ a = A–H …..B covalent A—H bond
ψ b = A-__H+ …..B ionic A—H bond
ψ c = A-–H …..B+ charge transfer, A…B bond
ψ d = A+__H- …..B ionic A—H bond
ψ e = A–H- …..B+ charge transfer, H…B bond
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His conclusion is that for O–H ….O bonds at O….O = 2.8 A°, the sum of the
electrostatic components, ψ b + ψ d has contributed about 65% of the hydrogen bond
energy. As the O….O distance becomes closer the quantum mechanical charge transfer
contributions became more important, while for longer weak bonds, the interaction
became more electrostatic. Later ab-initio molecular orbital calculations supported this
general chemical concept. As a criterion, it is reasonable to assume that if the vibration
frequency and the bond length of the covalent A—H bond are not significantly altered
on the hydrogen bond formation, the major component of the bonding is electrostatic.
Computers make it possible for ab-initio molecular orbital methods to explore
the potential energy for hydrogen bonded dimers with a variety of likely configurations
and conformations. The lowest energy minima are sought with the structures and
energies associated with them. Decomposing these energies into components, known as
the Morokuma [8] decomposition method, subdivide the total bonding energy into
electrostatic (es), polarization (pl), exchange repulsion (ex), charge transfer (ct), and
coupling (mix). The electrostatic component includes monopole-monopole (r-1),
monopole-dipole (r-2), dipole-dipole (r-3) terms and higher combinations of classical
interactions between undisturbed monomer charge distributions. The electron
distributions of the molecules are disturbed by the close approaches due to the hydrogen
bonding. This gives rise to polarization and quantum mechanical interactions, exchange
repulsion, charge transfer, and dispersion. The polarization is the effect of the distortion
of the electron distributions of A—H by B and B by A—H. This is a stabilizing
interaction. The exchange repulsion is the short-range repulsion of the electron
distributions of the donor and acceptor groups. It accounts for the overlap of charges in
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occupied orbitals of both donor and acceptor. Charge transfer is the result of the transfer
of electrons between occupied orbitals on the donor to vacant orbitals on the acceptor
and vice versa. The coupling term allows for the fact that these four interactions are not
strictly independent of each other. It is small, except for large bonding energies. The
electrostatic, polarization, and charge transfer are attractive at equilibrium distances
while the exchange repulsion is balancing them.
Hydrogen bonds which involve two acceptors are termed as three centered
hydrogen bonds, since the hydrogen is bonded to three atoms: one by a covalent bond
and two by hydrogen bonds. Three center bonds are also described as bifurcated
hydrogen bonds. Sometimes, a bond occurs between two donor hydrogen’s and one
acceptor. Such bonds are known as Chelated hydrogen bonds. Chelated hydrogen bonds
and three centered hydrogen bonds are shown in Figure 1.2. The hydrogen bond plays a
significant role in most biological systems. In these systems, the donor and acceptor
atoms are either Nitrogen (N) or Oxygen (O).
Figure 1.2: Bifurcated Hydrogen bonds
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The discovery of hydrogen bonding between the bases adenine and uracil and
between guanine and cytosine by Watson and Crick together with the crystal structure
analyses of the proteins, hemoglobin and myoglobin by Perutz and Kendrew and their
co-workers, initiated a new science known as molecular biology [9]. Nearly all
biological molecules contain hydrogen bonds, which play a critical role for the function
of biological macromolecules and the maintenance of living organisms. The function of
biological macromolecules is closely associated with their structures. These structures
are stabilized by covalent bonds which link individual units such as amino acids,
nucleotides or sugars.
1.3 Types of Hydrogen bonds
Hydrogen bonds are classified into two types as,
(i) Intermolecular hydrogen bonds
(ii) Intramolecular hydrogen bonds
(i) Intermolecular hydrogen bonds
In such a type of linkages, two or more than two molecules of the same
compound combine together to give a polymeric aggregate.
Example: HF molecule
When a number of HF molecules are brought together, the positive end of one
dipole Hδ+—Fδ- attracts the –ve end of the other similar dipole, Hδ+
—Fδ- and these
molecules associated together to form a cluster, (HF)n shown as,
[ --- H δ+ – F δ- --- H δ+ – F δ- --- H δ+ – F δ- --- H δ+ – Fδ- --- ]n.
In the case of (NH3)n and (H2O)n molecules are associated together to form
clusters.
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(ii) Intramolecular hydrogen bonds
In this type, hydrogen bonding occurs within two atoms of the same molecule.
Generally, this type of bonding is known as chelation. This is possible in ortho
substituted compounds.
An important example of a molecule having intra molecular H — bonding is
furnished by o – nitrophenol. The o – nitrophenol boils at 214oC, while o — and o —
isomer boil at 290oC and 270oC respectively. Thus we see that o — form has the
minimum boiling point which is accounted for the assumption that o — form contains
an intramolecular (internal) H – bonding which can be represented as shown below.
Figure 1.3: Intramolecular Hydrogen bonding in o – nitrophenol
This type of intra – molecular H – bonding is not possible in m- and p- isomers
because of the size of the ring.
Thus in m- and p- forms inter – molecular H – bonding takes place and this
results in some degree of association among a number of m- and p- forms. It is this
association which accounts for the higher boiling points of m- and p- isomers.
1.4 Different Techniques to study Hydrogen bonds
The different techniques to study hydrogen bonded systems are broadly
classified into three categories, as follows:
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(i) Physical methods: Physical methods include the study of molecular weight,
melting/boiling points, viscosity, dielectric properties and effect of pressure and
temperature.
(ii) Spectroscopic methods: Spectroscopic methods normally used to study the
hydrogen bonded systems are Vibrational Infrared Spectroscopy (IR and Raman), NMR
spectroscopy, Electron absorption studies and Gas-Phase Microwave rotational
spectroscopy.
(iii) Diffraction Methods: Neutron and X-ray diffraction are used in the analysis of
hydrogen bonds.
(iv) Computational Chemistry: Molecular mechanics methods, semiempirical methods,
ab-initio methods and Monte Carlo simulations are some of the computational
techniques adopted to study hydrogen bonds.
1.4.1 Physical methods to study Hydrogen bonds
Hydrogen bonding can affect the physical properties of gases, liquids and solids.
In the absence of hydrogen bonding, the physical properties of gases, liquids and solids
are said to be normal. That means, they are predictable from laws that relate physical
properties solely to the chemical composition of the molecules involved. In gases,
deviations from Raoult’s law for ideal gas properties,
( )VP = RTMw
,
is an evidence but not proof of hydrogen bonding. In liquids, deviations from Trouton’s
Rule, Vap
bp
∆H
T≈ 22 cal deg-1mol-1 ,
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is also an evidence. In crystals, hydrogen bonding in hydrates increases molar volume
whereas in organic compounds, it tends to decrease molar volume, because
intermolecular hydrogen bonding generally brings the molecules into closer contact
than van der Waals interactions.
(i) Molecular weight measurements
All inter molecularly hydrogen bonded systems form associated species. These
species increase the apparent molecular weight of the system under study. The apparent
molecular weight is measured from vapour density measurements and the ideal gas
equation is used for this purpose. Since PV = nRT where P and V are the pressure and
volume of gas, n = w/M is the number of moles of gas, R is the gas constant, T is the
temperature, w is the weight and M is the molecular weight of the sample.
(ii) Melting and boiling point measurements
Melting and boiling points of inter-molecular hydrogen bonded species are
significantly higher than those compared to similar molecular weight non-hydrogen
bonded species. This is because of the extra energy needed to break the hydrogen bonds
present.
(iii) Viscosity measurements
Viscosity of hydrogen bonded systems increases due to the formation of
associated species. The excess values associated with molecular weights, melting and
boiling points and viscosity values are true only for inter-molecular hydrogen bonded
species only since intramolecular hydrogen bonding does not lead to association of the
molecules. Nikiforov et al. [10] have suggested an expression for calculation of the
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viscosity of binary liquid systems with different types of interaction between the two
constituent species.
(iv) Dielectric permittivity measurements
Although there is no direct theory to predict the dielectric permittivity spectra of
hydrogen bonded systems, a few observations are generally true. Almost all hydrogen
bonded systems show multiple relaxation process very often with distinct relaxation
mechanisms. The relatively high dielectric permittivity of hydrogen bonded liquids is at
least partially due to a high ratio of dipole/quadrupole moment of molecules that can
form hydrogen bonds. Also direct interactions (next neighbour interactions) are seen to
contribute less to the dipole angular correlation function than pair correlations due to
indirect interactions.
(v) Effect of pressure and temperature on Hydrogen bonded systems
Increase of pressure on hydrogen bonded system increases the number of
hydrogen bonds since more number of molecules come closer. Hydrogen bond
formation is exothermic whereas hydrogen bond breaking is endothermic. This implies
that the hydrogen bonded systems have lower energy than the sum of energies of the
non-bonded individual components. It is therefore possible to study the
thermodynamics of hydrogen bond formation either directly or by measuring the
equilibrium constants of any property that changes with temperature or concentration in
an inert solvent.
The major demerit associated with the physical methods is that none of them can
prove the existence of hydrogen bonds uniquely. They also cannot locate the exact
position of hydrogen bond in the system. These limitations can be overcome in
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spectroscopic techniques where the site of hydrogen bond and very often the number of
hydrogen bonds present in a system can be determined.
1.4.2 Spectroscopic studies of Hydrogen bonds
(i) Vibrational spectroscopy
A vibrational spectrum of hydrogen bonded system (A-H…B) in the IR region is
used as an important tool. The prominent features of the IR spectra of hydrogen bond
system are
a) Shift of fundamental A-H bond (ν (A-H)) to lower frequencies. This is because
hydrogen bond renders the A-H bond weaker in the pure state.
b) Shift of A-H in plane/out of plane deformation to higher frequencies,
c) Formation of a new A-H….B fundamental stretch/deformation band at lower
frequencies and
d) Change in the bond contour and integrated absorption intensities are also observed
but are very often difficult to analyze quantitatively
IR spectroscopy can identify the presence of monomers, dimers, trimers etc. in
species containing OH bonds. The broad OH band can be deconvoluted into the
individual monomer, dimer, trimer etc. bands since the full width at half maximum and
peak positions of these bands are all known. The overlap of water OH band with that of
the alcohol band makes it necessary to record the spectra under vacuum conditions so
that the sample does not absorb water.
(ii) NMR Spectroscopy
NMR spectroscopy is another extremely sensitive method for identifying
hydrogen bonding. NMR spectroscopists observe the chemical shift caused by the
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change in the electronic environment around the proton. It is less widely applied than
infrared spectroscopy because of the complexity of hydrogen bonding in solution. The
development of solid state NMR through the 13C - cross polarization magic angle
spinning methods (CP-MAS) and the more difficult 1H - combined rotational and
multiple pulse spectroscopy (CRAMPS) provides very effective tools for studying
hydrogen bonding in solids. The development of multidimensional methods has made
NMR spectroscopy a powerful tool for elucidating molecular structure in solution.
Nevertheless, it shows relatively little impact on the study of hydrogen bonds.
(iii) Electron absorption spectroscopy
This is useful technique if the hydrogen bond part of the molecule has
absorption in the visible/UV region (6000-2000 A0) of the electromagnetic spectrum.
The hydrogen bond acceptor (A-H) shows a blue shift and the hydrogen bond donors
show a red shift in the hydrogen bond species.
1.4.3 Diffraction studies of Hydrogen bonds
Diffraction methods depend on the three-dimensional periodicity of the atoms.
The ability of atoms to diffract or scatter x-rays is proportional to the atom’s electron
density. Hydrogen with one electron has a very small ability for diffraction. Therefore,
this method cannot be used to determine the position of hydrogen.
Single crystal structure analysis has a special role in the study of hydrogen
bonds as they provide direct information concerning the stereochemistry. Location of
hydrogen atoms is essential to understand the nature of the hydrogen bond. Strong,
moderate and weak bonds can be distinguished using crystal structure analysis and
infrared spectroscopy. Electron diffraction and X-ray diffraction of liquids do not
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provide information concerning the position of the hydrogen atoms and therefore they
are not used for the study of hydrogen bonds.
Neutron diffraction is a better tool to study hydrogen bonds since proton has a
larger cross section to scatter neutrons. The coherent scattering of the atoms is not a
function of atomic number like X-ray, but varies over a relatively narrow range.
Hydrogen has a scattering power for neutrons that is equivalent to half that of carbon
and oxygen while it is 1/50 compared to X-rays. This very large incoherent neutron
scattering factor for hydrogen helps us to derive information concerning the positions of
hydrogen atoms with accuracy up to a third decimal point in A0. But, this involves a
greater commitment in time and money. Corrections for thermal motion and
anharmonic stretching motion should be taken into consideration, if precise
comparisons are made between the hydrogen bond geometries at different temperatures,
or with theoretical calculations, which predict the geometries of molecules at rest.
It is important to realize that different methods provide atomic structure and
processes on different time scales. Spectroscopic methods provide information relating
to structure and processes on a picosecond’s time scale (10-10-10-15sec). Diffraction and
thermodynamic methods are at the other end of the timescale (10-103sec). Crystal
structure analysis gives an average structure with respect to both time and space. NMR
spectroscopy is midway at 10-10-4sec. Therefore, the spectroscopists and diffractionists
sometimes view hydrogen bonded structures from different perspectives and their
conceptual models do not always correspond to each other.
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1.4.4 Computational molecular modeling
Computational molecular modeling has matured with the development of
computers. The main requirement is that the model must correspond to what the
experiment says. Two approaches have been employed viz: molecular mechanics and
quantum mechanics.
(i) Molecular mechanics
Molecular mechanics has many facets in common with early molecular
spectroscopy, which applied methods of classical mechanics to an understanding of
vibrational and rotational spectra. Atoms are treated as hard spheres with fixed masses
connected by springs with assigned bending and stretching force constants. Electrostatic
attractions and repulsions are also included. The sum of the equations taking all these
factors into account is called the “force field” and molecular mechanics calculations are
sometimes called force field calculations. A number of force fields have been developed
in recent years, each with a slightly different set of basic assumptions and
approximations. A simple force field can be Etotal = Estretch + Ebend + Etorsion + Enonbonded.
When two atoms A and B approach each other, resulting in the formation of the
diatomic molecule A—B, the energy of the system follows the Morse Potential energy
plot. As A and B approach from infinity, the electrons around each nucleus become
polarized, leading to an attractive London or dispersion force. The energy continues to
decrease until the equilibrium bond length ro is reached. At shorter bond lengths,
repulsion between electron charge clouds becomes increasingly effective and the energy
quickly mounts due to van der Waals repulsion.
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Bond stretching forces may be represented by a Hooke’s law type of interaction,
Estr = kstr ( r- ro )2 / 2. Similarly the bending interactions may be approximated by Ebend =
kbend (θ - θo)2 / 2 and Etorsion = V F (sin ω), where ω is the angle of rotation. The final
term in the simplified force field is a nonbonding interaction reflecting the London
forces that are attractive at long distances and are counterbalanced by van der Waals
forces of repulsion. The bond is formed at a distance between the atoms as they
approach the sum of their van der Waals radii. This term has been represented in a
number of different forms by the developers of various force fields. Many of these start
with Leonard Jones potential
Enonbonded = e [ (ro/ r)12 — 2 (ro/ r)6 ]
where e is the energy value at the minimum in the curve coinciding with ro. Each force
field defines a mechanical model to be used in computing the molecular structure and
its accompanying energy.
Once an input structure has been entered into the program, a set of parameters
describing the molecular geometry is computed i.e., bond lengths, bond angles and
torsion angles. These values are then fed into the terms of the force field equation and a
steric energy is calculated. Generally, this energy is expressed in kcal/mol and is the
sum of all the potential energy terms contained in the force field. The steric energy is a
value specific for a given force field. Such numbers cannot be used to compare values
calculated by other programs. Furthermore, as a generalization, such steric energies
cannot be used to compare the relative stabilities of different molecules, though they
may be used to compare different conformations of the same molecule. To alter the
structure of the molecule in a systematic way to minimize the steric energy, a variety of
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mathematical approaches such as Newton-Raphson method are used. The minimum
energy structure obtained in this way is the nearest local minimum on the potential
energy surface of the molecule.
Molecular mechanics are used primarily to simulate the structures of larger
molecules and macromolecules. It can also be extended to arrays of hydrogen bonded
molecules. Molecular dynamics, which uses the same force fields, permits the
exploration of transitions between different conformations separated by torsional energy
barriers. It can also be extended to clusters of molecules and to simulate the effect of
solvation.
(ii) Quantum-mechanical calculations
Calculating the structure, energies and other physical properties of an assembly
of nuclei and electrons in molecules is an extraordinarily complex problem.
Nevertheless, an ab initio quantum mechanical calculation has become an important
method for understanding hydrogen bonding. After 1980, it has become possible for
theoreticians to aim for computations for simple dimers, which reproduced the binding
energies within the uncertainties of the experimental measurements.
a) Ab-initio calculation
Ab-initio molecular orbital theory is concerned with predicting the properties of
atomic and molecular systems. This method bases on the fundamental laws of quantum
mechanics and uses a variety of mathematical transformation and approximation
techniques to solve its fundamental equations.
The ab-initio methods seek minimum energy intra molecular and inter molecular
geometry by solving the wave equation using a Linear Combination of Atomic Orbitals
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known as the LCAO approximation. Coefficients are attached to each of the electron
atomic orbitals and varied to obtain the minimum energy. Polarization effects are
introduced by adding p-orbitals to the s-orbitals of hydrogen, d-orbitals to the s and p
orbitals of first row elements and so on. To facilitate the calculations, these atomic
orbitals have to be expressed in an analytical form using basic functions. This is known
as the basis set. When the basis set becomes more sophisticated, the computing time
needed increases considerably. The computing time is roughly proportional to the fourth
power of the basis functions. The greater the basis set, the closer the result comes to the
Hartree-Fock limit. However, sometimes simpler basis sets give better agreement with
experimental data due to cancellation of errors.
b) Semiempirical methods
In the late 1970s, interest in the development of semiempirical methods has
grown in the laboratories of M.J.S. Dewar at the University of Texas, as the ab-initio
methods are extremely time costly. No integrals are evaluated in these semiempirical
methods. When the integral values are required, experimental numbers are used in their
places. The parameterization is then tested against a limited set of molecules to ensure
its accuracy. Like the numeric iteration, this process is a continuing one, until
convergence in energy parameters is achieved. AM1 (Austin Model 1) is a recent model
parameterized to treat hydrogen bonds successfully.
(iii) Monte Carlo simulations
The Monte Carlo method of searching uses probability theory. This method is
different from other molecular dynamic methods in solving the Newtonian equations of
motion without generating successive configurations. Instead, a starting configuration is
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subjected to a random series of changes in atomic positions. Each new configuration is
minimized, and the potential energy is compared to that of the preceding structure. If
the new energy is less, the structure is kept. If the new energy is greater, it is discarded
except in the case where the term exp (-∆V/kT) is less than a randomly chosen number
between 0 and 1. Monte Carlo methodology is important in studying large protein and
polymer molecules that are not readily studied by molecular mechanics. Monte Carlo
methods are often used to compute thermodynamic quantities by statistical
thermodynamics.
1.5 Chosen systems for the present study
In the present work, the following compounds are chosen in order to study the
hydrogen bonding between two individual systems. These compounds of AR grade are
purchased from E. Merck, Germany and are purified by standard methods. The
formation of hydrogen bond between the individual systems is conformed from the
experimental FT-IR spectra in the range 400 cm-1 - 4000 cm-1 by using the Perkin Elmer
FT-IR spectrometer. The experimental dipole moment and FT-IR values are compared
with the theoretical quantum mechanical calculations and are in reasonable agreement
with one another within the error limits. Further, various dielectric and thermodynamic
parameters are studied on these systems to analyze the formation of hydrogen bonding.
The chosen pure systems (monomers)
Alcohols
1) Propan-1-ol
2) Propan-2-ol
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Aromatic amines
3) Aniline
4) N-Methyl aniline
Physical and chemical properties of the pure systems
Alcohols
Alcohols play an important role in many chemical reactions due to their ability
to undergo self-association with manifold internal structures. They are in wide use in
industry and science as reagents, solvents and fuels. Liquid alcohols represent a
favorable system for evaluating the importance of both hydrophilic and hydrophobic
interactions in determining the relevant properties of the liquid phase. A substantial
effort is being taken to understand the mechanisms of molecular interactions associated
with the hydrogen bonds. The knowledge and understanding of the molecular
mechanisms in systems containing alcohols constitute a valuable source of information
for theoretical as well as experimental analysis of the same mechanism in more general
systems.
1. Propan-1-ol
Propan-1-ol is a primary alcohol with molecular formula C3H8O. It is also
known as 1-propanol, 1-propyl alcohol, n-propyl alcohol, n-propanol or simply
propanol with abbreviation 1PN. It is formed naturally in small amounts during several
fermentation processes. It is an isomer of propan-2-ol. It is used as a solvent in the
pharmaceutical industry, and for manufacturing resins and cellulose esters.
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2. Propan-2-ol
Propan-2-ol is a secondary alcohol with molecular formula C3H8O. It is also
known as isopropyl alcohol and 2-propanol with the abbreviation IPA. It is a colorless,
flammable chemical compound with a strong odor. In this compound, the alcohol
carbon is attached to two other carbons sometimes shown as (CH3)2CHOH. As a
biological specimen preservative, propan-2-ol provides a comparatively non-toxic
alternative to formaldehyde and other synthetic preservatives. Propan-2-ol solutions of
concentration 90-99% are optimal for preserving specimens, although concentrations as
low as 70% can be used in emergencies.
Aromatic amines
An aromatic amine is an amine with an aromatic substituent i.e., –NH2, –NH or
nitrogen group(s) attached to an aromatic hydrocarbon, whose structure usually contains
one or more benzene rings. Amines are used as a starting material for the manufacture
of azo dyes. It reacts with nitric (III) acid to form diazonium salt, which can undergo
coupling reaction to form azo compound. As azo-compounds are highly coloured, they
are widely used in dyeing industries.
3. Aniline
Aniline is colorless, resinifies in air and slowly oxidizes, giving a red-brown tint
to aged samples. Aniline or amino benzene is an organic compound with the chemical
formula C6H7N. It is the simplest and one of the most important aromatic amines, being
used as a precursor to more complex chemicals. Its main application is in the
manufacture of Polyurethane. It possesses an unpleasant odour of rotten fish.The great
commercial value of aniline was due to the readiness with which it yields, directly or
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indirectly, dyestuffs. In addition to its use as a precursor to dyestuffs, it is a starting
product for the manufacture of many drugs, such as paracetamol. When polymerized,
aniline can be used as a type of nanowire for use as a semiconducting electrode bridge,
most recently used for nano-scale devices such as biosensors. These polyaniline
nanowires can be doped with a dopant accordingly in order to achieve certain
semiconducting properties.
Table 1.1: Physical and Chemical properties of the pure systems.
Property Compound
Propan-1-ol Propan-2-ol Aniline N-methyl aniline
Molecular
formula
C3H8O C3H8O C6H7N C7H9N
Molar Mass 60.10 g mol-1 60.10 g mol-1 93.13 g mol-1 107.16 g mol-1
Appearance Colorless liquid Colorless liquid Colourless liquid Pale Yellow to
brown Liquid
Density 0.7955 g/cm3 0.785 g/ cm3 1.022 g/ cm3 0.985 g/ cm3
Melting Point 147 K 184 K 266.7 K 216 K
Boiling Point 370.1 K 355.5 K 457.13 K 469 K
Dipole moment
(gas) [11]
1.66 D
1.66 D
1.53 D
1.67 D
Molecular
Structure
(Red: Oxygen, Black: Carbon, Blue: Nitrogen and White: Hydrogen)
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4. N- Methyl aniline
N-Methyl aniline is a secondary amine in aniline class, with the chemical
formula C7H9N and it is abbreviated as NMA. This compound is Pale Yellow in colour.
It is used as a latent and coupling solvent. Its important use is as an intermediate in the
manufacturing of dyes, agrochemicals, pesticides, antioxidants and other organic
products.
The binary systems in the present study are:
1) Propan-2-ol + Aniline (system 1)
2) Propan-1-ol + N-Methyl aniline (system 2)
3) Propan-2-ol + N-Methyl aniline (system 3)
1.6 Literature survey
Most of the dielectric relaxation processes reported in the literature were studied
for dilute solutions of polar substance in non-polar liquids. The non-polar liquid does
not itself undergo relaxation but alters the relaxation time of the solute molecule by
reducing the internal field and changing the viscosity. Relatively little work has been
done on mixtures of polar components. The study of dielectric dispersion and
absorption of a binary liquid mixture provides a very sensitive tool for detecting
molecular interactions. The formation of complexes or presence of association leads to
relaxation times considerably higher than those for the uncomplexed (unassociated)
species. In many cases, the interaction between the constituents may not be sufficiently
strong to allow the formation of stable complex. Even in such cases, the relaxation time
has increased because of the increased resistance to rotation due to attractive influence
of neighboring molecules. It has been shown by Schallamach [12] and others [13] that a
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binary liquid mixture in which both the components are either associated or non
associated shows a single relaxation time, whereas two distinct relaxation times are
observed in mixtures with one associated and other non associated components.
Crossley et al. [14], Garg and Smyth [15] have studied the complex dielectric
permittivity of six isomeric octyl alcohols at different concentrations in a non polar
solution i.e., n-heptane at different microwave frequency ranges and showed the
existence of three relaxation process in alcohols, dominated by low frequency Debye
type process with a single relaxation time. The three dispersion regions of the pure
primary alcohols are described by a short relaxation time 3τ which is attributed to
hydroxyl group rotation, an intermediate relaxation time 2τ which is attributed to the
orientational motions of the small molecular species and the dominant long relaxation
time 1τ which is associated with the hydrogen bonded structure in the liquid.
Kadaba [16] observed that the concentration dependence of the Kirkwood
correlation factor showed a minimum at particular concentration while diluting alcohols
with non polar solvents. This means qualitatively that dilution leads to the formation of
cyclic multimers with anti parallel dipoles.
Winklemann [17] has developed comprehensive relations linking the complex
permittivity of binary mixtures with short range and long-range interactions as an
extension of Kirkwood theory.
The theoretical approach and computer simulation study were reported to
understand the relaxation behavior and the hydrogen bonding in alcohols by Minami et
al. [18] and Padro et al. [19].
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The dielectric relaxation of mixture of water and primary alcohols using time
domain reflectometry technique was studied by Satoru Mashimo et al., [20].
Brilliantov et al. [21] has explained the continuum theory of the rotational
motion of a uniformly charged solute molecule in a non polar solvent which takes into
account dielectric friction, dielectric saturation and spatial dependence of the solvent
viscosity due to electrostriction.
The characteristic features of the supramolecular clusters which comprise the
QCE (Quantum Cluster Equilibrium) model are discussed in terms of binding energies
and geometries of alcohols by Huelsekoph and Ludwig [22].
Schwerdtfeger and Kohler [23] have studied the relaxation behavior of
monohydric alcohols with n-alkanes where as Hiejima and Yao [24] have studied
dielectric relaxation behavior of alcohols in fluid phase.
Conformational and dielectric analysis of hydrogen bonded polar binary
mixtures of methyl benzoate and N-methyl aniline have been reported by Chitra et al.,
[25].
Chaudhari et al. [26] have studied the parallel and anti parallel alignment of
dipoles with the variation of excess permittivity and rotation of dipoles with excess
inverse relaxation time.
Kaatze et al. [27] has explained the wait-and-switch model of dipole
reorientation in hydrogen bonded systems of alcohols in a polar and non-polar medium.
A car-Parrinello molecular dynamics simulation has been performed on fully
deuterated liquid methanol by Paglai et al. [28]. The results are compared with the latest
available experimental and theoretical data. It has been shown that the liquid aggregates
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in chains of hydrogen bonded molecules. The structure of the aggregates is
characterized and it is found that the dynamics include fast and a slow regime. The
weak H- bond formed by the methyl group hydrogen and oxygen atoms of surrounding
molecules has been characterized.
Handgraal et al. [29] has explained the density-functional theory based
molecular dynamics of methanol. The structural, dynamical and electronic properties of
liquid methanol under ambient conditions were analyzed in the study.
Pawar et al. [30] have reported the temperature-dependent dielectric relaxation
study of chlorobenzene with n-methylformamide from 10 MHz to 20 GHz using time
domain reflectometry (TDR) in the temperature range 15 °C to 45 °C for 11 different
concentrations of the system.
Madhurima et al. [31] have studied the effect of steric hindrance of ketones in
the dielectric relaxation of binary mixtures of methanol and ketone and compared the
experimental and theoretical dipole moment values obtained from quantum mechanical
calculations such as ab initio and semiempirical methods.
Thenappan et al. [32] have studied the nature of intermolecular interactions
between associative and non-associative polar liquids and explained the formation of
cyclic and linear α-multimers in the binary mixtures.
Dharmalingam et al. [33] have studied the solute-solvent interaction which
explains the ordering of the molecules in the liquid phase with the help of Kirkwood
correlation factor, and also explained the creation of dimers and monomers with the
help of excess thermodynamical parameters.
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Parthipan et al. [34] have studied the thermodynamical parameters of anisole
with 2-ethyl-1-hexanol and decyl alcohol to analyze the heteromolecular interactions.
Pang et al. [35] have measured the densities and viscosities of binary aqueous
solutions of 1-propanol and 2-propanol over the whole composition range at
temperatures between 293.15 K and 333.15 K. The activation free energies for viscous
flow for aqueous solutions of 1-propanol and 2-propanol were calculated and found to
be 17.94 and 22.16 kJ mol− 1, respectively.
Jimenez et al. [36] have studied the electromagnetic behaviour of polar and non-
polar dielectric mixtures of the type alcohol + n-alkane from primary alcohols 1-
pentanol, 1-hexanol, and 1-heptanol using TDR technique.
Vishwam et al. [37] have studied the formation of hydrogen bond between
propionaldehyde and isopropylamine from experimental and theoretical calculations
and explained the absence of ionic structure from excess dipole moment values.
Aparicio et al. [38] have determined the microwave dielectric spectroscopy of
methyl benzoate in different alkenes and alcohols and explained the different dielectric
parameters for the conformation of hydrogen bond.
Lone et al. [39] used the bilinear calibration method to obtain the dielectric
parameters viz., static dielectric constant ( 0ε ) and relaxation time. Using these
parameters the values of excess permittivity ( Eε ), excess inverse relaxation time,
Bruggeman factor ( Bf ) and thermodynamic parameters are determined.
Kinart et al. [40] have measured the densities and relative permittivities, at
T = (293.15, 298.15 and 303.15) K, in the binary mixtures of 2-ethoxyethanol with
ethylene glycol, diethylene glycol, tri ethylene glycol and tetra ethylene glycol as a
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function of composition. From the experimental data the excess molar volumes and
deviations in the relative permittivity have been calculated. The results are discussed in
terms of intermolecular interactions and structural properties of studied binary mixtures.
Yomogida and Nozaki [41] have done the Complex permittivity measurements
on acetophenone and its derivatives o-hydroxy benzaldehyde, o-methyl acetophenone,
and o-hydroxy acetophenone are performed at frequencies between 1 MHz and 20 GHz
at temperatures from 273 to 323 K. The parameters obtained from the fitting of the
complex permittivity are analyzed in order to study the effects of the hydroxyl group
within a molecule on the dielectric relaxation phenomenon in these liquids. The analysis
indicates that dynamical properties are affected not only by the intermolecular hydrogen
bond but also by the slight change in molecular structure.
Lileev and Lyashchenko [42] have measured the high-frequency dielectric
permittivity, dielectric losses and low-frequency conductivity of aqueous solutions of
ammonium salts at 288, 298 and 308 K. These values characterize the mobility of water
molecules in solutions.
Prajapati et al. [43] have reported the dielectric relaxation and dispersion studies
of mixtures of 1-propanol and benzonitrile in pure liquid state at radio and microwave
frequencies and explained the molecular interaction between the molecular species of
the liquid mixtures with the dielectric parameters.
Yomogida et al. [44] have reported the complex permittivity measurements of a
series of normal alcohols (methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol)
which are performed by THz time-domain spectroscopy in the frequency range of 0.2–
2.5 THz at temperatures of 253–323 K.
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Ghanadzadeh Gilani et al. [45] have made the dielectric measurements of the
binary polar mixtures of the butanediols with 2-ethyl-1-hexanol (2EH) for various
concentrations at 298.2 K to study the Kirkwood correlation factor, Bruggeman factor
and excess permittivity.